Raster type printers, which have been implemented with various print engines commonly found in the arts, such as electro-photographic print engines and ink jet print engines, employ half-toning to transform continuous tone image data to print data that can be printed as an array of dots that can be of substantially similar size. For example, 24 bit/pixel continuous tone image data can be half-toned to a plurality of single color one-bit per pixel bitmaps.
For printing with multiple colors, half-toning presents a particular challenge. For dot-on-dot printing, in which printed locations are printed with one or more dots, a single half-toning screen can be used. For instance, a field of 10% blue would have 10% of locations printed with cyan and magenta ink, while 90% of locations remain unprinted. This has the disadvantage of reduced spatial frequency with respect to methods that distribute dots to different locations. This also tends to give the appearance of darker dots more widely spaced apart, producing a grainy image. The same can be said for clustered dot printing techniques in which different color dots may be printed adjacent to each other or otherwise clustered to create a multi-dot cluster that reads as an intermediate color. Accordingly, it is desirable to print the individual dots at closely spaced separate (non-overlapping) locations, relying on the viewer's eye to integrate the different color dots into the intended color.
It can be difficult to achieve substantial uniformity or even distribution of the half-toned dots in dot-on-dot printing devices. Substantial uniformity can be computationally expensive.
One cause of half-tone pattern graininess is spatial luminance variation. A basic property of the color map which is advantageous to control is how the light level, or luminance, changes throughout the color map. Luminance is a photometric quantity which, in essence, is the effect of radiance on our eyes. Radiance is the physical quantity related to light intensity, i.e., the power of the light spreading out in some solid angle over an area. Luminance is a single scalar, (i.e., the integration of radiance weighted with a curve), which describes how efficiently different wavelengths of light trigger cone receptors in our eyes. Because luminance is a weighted integral of radiance, a linear relationship exists between the two. Brightness, on the other hand, is the subjective visual experience of luminance. Roughly, it is the effect of luminance on the brain. Because its a subjective quantity, the relationship of brightness to luminance is non-linear, approximately a cube root.
For multi-function device applications, color error diffusion is a popular half-toning method for copy application, due to its excellent detail preservation and moiré resistance. Color error diffusion can be classified into two types, namely, vector error diffusion and scalar error diffusion. Compared to scalar error diffusion, vector error diffusion is often considered to be better in image quality because vector error diffusion often achieves higher half-tone quality compared to the channel-independent scalar color error diffusion due to the better correlation among CMYK color plane. However, it requires significantly more computation, and can introduce artifacts due to accumulation of the errors in output device space. Vector error diffusion can be slow to diffuse error in the image during the half-toning process due to error accumulation.
Furthermore, half-toning in a CMYK domain does not necessarily result in a well-behaved inter-color correlation among secondary colors (RGB). Consequently, noticeable increases in graininess can be observed when significant amounts of secondary color dots are used to achieve the input ink coverage. Therefore, explicit secondary color dot control is desirable to achieve uniform dot distribution. It is also desirable to position the color dots uniformly in the very highlight and very dark shadow areas due to a requirement of long-distance spatial correlation.
Accordingly, what is needed in this art are increasingly sophisticated applications and advanced methodologies which perform color error diffusion in a color management system.